In the ocean engineering field, no matter from the points of economy of oil and gas production in shallow water areas and necessity of oilfield development in deepwater and superdeep water areas, corresponding scale of floating structures are required to be used. Besides, with the trends of multifunctional integration and development toward deep sea, the floating structures not only need enough hold capacity and deck area to bear oil and gas and equipment necessary to develop and produce, but need favorable movement performance to bear the combined action of severe environment loads from wind, wave and current in deep sea. The existing floating structures mainly include:
a semi-submersible floating structure is a multi-body small-waterplane mobile platform residing in that most floating body is submerged in water, comprises a platform body, pillars and a lower body or buoyancy tank, can be kept at a stable position basically after being moored and has relatively small movement amplitude under the environment load. However, owing to limited bearing capability and relatively small hold capacity space, the semi-submersible floating structure will limit storage of large-scale oil and gas and installation of essential equipment and produce the problem of uneven load distribution caused by occupancy to a large amount of deck area even though a method of configuring an oil tank is adopted. Therefore, it is very difficult for the traditional semi-submersible floating structure to realize integration of large-scale oil and gas production, storage, production, processing and transport.
Most of SPAR single-pillar floating structures are float bowls. The main body is of a single-cylinder structure and is perpendicularly suspended in water, thus being particularly suitable for deepwater operations. This kind of structures can be configured for deepwater oil and gas development and have movement stability and favorable safety, however, its shortages are that deep draught is greatly affected by ocean current, the structures are very difficult to haul and volumes for accommodating equipment and storing oil and gas and the deck area are very small, all of which limit the develop of multifunctional integration of such structures on different levels.
The traditional floating production storage and offloading system (FPSO for short) systems are generally transformed from old oil tankers or are barge-like FPSO systems designed and built in accordance with standard shipping ideas. At present, the techniques are relatively mature and can be used for exploitation, storage and production of large-scale oil and gas. However, those ship-like floating bodies have some limitations and shortages in the hydrodynamic performance as follows:
the natural heave period of the traditional ship-like FPSO is hardly away from a wave energy concentration area, and the heave movement amplitude is relatively large. Besides, the ship-like FPSO is very sensitive to the action direction of waves and has overlarge area of transverse motion in heading sea, thus resulting in relatively poor sway movement performance, all of which will seriously affect normal work of various equipments and instruments of FPSO as well as quality of produced raw oil and comfortable level of staff. The traditional ship-like FPSO has a 360-degree all-sided freely-rotating wind indicator effect due to a single-point mooring system equipped with an inner turret and a fluid connector, however, relatively serious yaw movement will not only affect normal running of many works, but will wear the inner turret and the fluid connector to make them frequent in need of repair and maintenance. Therefore, the production cost will be greatly increased due to self exorbitant prices of the inner turret and the fluid connector and existence of potential downtime.
Moreover, the cylindrical FPSO main body is of a floating cylindrical structure moored at the bottom of the sea. Such structure has large-scale storage and oil and gas production capabilities, is not sensitive to the directivity of wind, wave and current and has relatively small yaw movement amplitude, and whereas it has many shortages in terms of performances: large heave movement amplitude of the floating body, the vortex-induced vibration and the living space is too close to the working space by reason of relatively small deck area, which is not advantageous to separating a danger area from a non-danger area.